The study of comparative anatomy predates the modern study of evolution. Early evolutionary scientists like Buffon and Lamarck used comparative anatomy to determine relationships between species. Organisms with similar structures, they argued, must have acquired these traits from a common ancestor. Today, comparative anatomy can serve as the first line of reasoning in determining the relatedness of species. However, there are many hidden dangers that make it necessary to support evidence from comparative anatomy with evidence from other fields of study.
A major problem in determining evolutionary relationships based on comparative anatomy can be seen when we look at a commonly found structure: the wing. Wings are present in a number of very different groups of organisms. Birds, bats and insects all have wings, but what does this say about how closely related the three groups are? It is tempting to say that the three groups must have had a common winged ancestor. However, were you actually to take the bait and say it, you would be wrong. Dead wrong. The wings of bats and birds are both derived from the forelimb of a common, probably wingless, ancestor. Both have wings with bone structures similar to the forelimbs of ancestral and current tetrapod, or four-legged, animals. Such traits that are derived from a trait found in a common ancestor are called homologous traits. Structurally speaking, though, the wings of bats and birds have little in common with those of insects. Bird wings and insect wings are an analogous trait, or a trait that has developed independently in two groups of organisms from unrelated ancestral traits.
Another difficulty in comparing traits between species rests on the fact that homologous structures not present in the adult organism often do appear in some stage of embryonic development. In this way, the embryo serves as a microcosm for evolution, passing through many of the stages of evolution to produce the current state of the organism. Species that bear little resemblance in their adult form may have strikingly similar embryonic stages. For example, in humans, the embryo passes through a stage in which it has gill structures like those of the fish from which all terrestrial animals evolved. For a large portion of its development the human embryo also possesses a tail, much like those of our close primate relatives. This tail is usually reabsorbed before birth, but occasionally children are born with the ancestral structure intact. Tails and even gills could be considered homologous traits between humans and primates or humans and fish, even though they are not present in the adult organism.